Power switching apparatus with open-load detection, referred to as a ‘smart power switch’, is used in many applications, including automotive equipment, especially to control vehicle lighting, for example. The present invention is particularly, but not exclusively, usable in such applications.
Integrated smart power switches use different strategies to detect the disconnection of an output load.
The simplest strategy is to monitor the status of the output during the OFF-state. In a known system of this kind, a very small test current is fed to the output line and the voltage across the load is monitored by a voltage comparator. With a normal load, the low impedance of the load ties the output voltage close to ground level (in the case of a high-side switch), in case of load disconnection or open circuit the output is tied to high voltage level by the current source.
The disadvantage of this solution is that it is not acceptable for detection of open-load during the ON-state of the switch, which is a particular disadvantage in many applications, such as switching automotive lighting, for example incandescent bulbs or light-emitting diodes (‘LED’s). To generate an information using this type of solution during the ON-state, the load must be switched OFF temporarily, which is visible as a flickering of the lamp. Even measurement during the OFF-state of the load can only be performed when the output is energised at least at low current levels but which may also generate unacceptable visible light, especially when using LED light sources.
Another technique to monitor a load disconnection or open circuit condition is to measure the load current in the switch. This can be done with a dedicated magnetic coupler or a shunt resistor based current measurement circuit. However, such circuits introduce not only additional cost and space requirements but also an additional voltage drop.
Another solution to measure the load current in the switch, especially where the switch comprises a MOS transistor is to mirror the output current during the ON-state by a regulated current source. This common and effective solution suffers mainly from a poor precision of the circuit recopy at small output currents. When the voltage drop across the switch is of the same magnitude as the offset of the regulating amplifier the error of the circuit can exceed 100% and adequate current measurement is no more possible.
Fast open-load detection is often required during ON state to get a quick reaction in case of dead bulbs on a car for example in order to be able to react by substituting emergency lighting for the normal lighting with a minimum of disruption. The open-load current detection threshold must be lower than the minimum acceptable current for a normal load. In the case of a standard driver used to drive a wide spread of loads this minimum current becomes difficult to measure. For example: A MOSFET (metal-oxide field-effect transistor) switch driver can be designed in terms of the drain-source resistance Rdson in the On-state to drive a 27 Watts incandescent bulb (Rdson: 40 mOhm typical, 20 mOhm minimum, for DC current of 2 amps from a nominally 12 volt power supply).
However, incandescent bulbs are more and more being replaced by an array of LEDs with minimum current around 100 mA. It is desirable for the same switch and driver to be used for both incandescent bulbs and LEDs so that the openload threshold current has to be adjusted below this 100 mA. Measuring 100 mA on a minimum Rdson of 20 mOhm (during ON state) means an absolute signal of 2 mV to be measured in a harsh environment (fast transients on supply), which is simply not a realistic solution in most applications of this kind.